There is no real right or wrong answer when it comes to Shepherd Hut insulation, and it is of course dependant on the roof material you use.
For both felt and shingles in is better to apply the insulation (flexible foil type) to the outside of the roof, then a flexible ply sheet fixed to the roof boards and the final roof covering to that.
If you have source yourself corrugated metal roof sheet it is best to fix felt to the roof as standard, then batten the roof, insulation between the battens and fixed and then the metal roof.
However, you can also batten the roof and apply the insulation, then a flexible ply with felt on top and then the metal roof.
Alternatively you could also insulate under the roof but you then run the risk of condensation between the roof and insulation, you would also remove the attractive hoops in the roof with this method.
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